FIG. 7 to FIG. 10C illustrate gas laser oscillators of the prior art. First, FIG. 7 shows an example of general structure of an axial-flow type gas laser oscillator of the prior art. In FIG. 7, discharge tube 701 made of a dielectric material such as glass is provided with electrodes 702 and 703 on the perimetric sides thereof. Electrodes 702 and 703 are connected to power supply 704. There is discharge space 705 formed inside discharge tube 701 between electrodes 702 and 703. Final stage mirror 706 having a surface of generally all reflection and output mirror 707 having a surface of partial reflection are securely placed to both ends of discharge space 705, and they constitute an optical resonator. Final stage mirror 706 and output mirror 707 are simply called mirrors. Arrow 709 represents a direction to which laser gas flows. The laser gas circulates inside the axial-flow type gas laser oscillator at a pressure of approximately 100 to 200 Torr. Heat exchangers 711 and 712 operate at all the time to lower temperature rise of the laser gas. Blower unit 713 circulates the laser gas to produce a flow of approximately 100 m/sec. in discharge space 705. Laser gas passage 710 and discharge tube 701 are connected with laser gas ports 714.
The laser gas delivered by blower unit 713 passes through laser gas passage 710, and it is introduced into discharge tube 701 from laser gas port 714. Electrodes 702 and 703 generate electrical discharge inside discharge space 705 under the above condition. The laser gas receives energy of the electrical discharge, and it is excited in discharge space 705. The excited laser gas turns into a resonant mode by the optical resonator composed of final stage mirror 706 and output mirror 707, and laser beam 708 is output from output mirror 707. This laser beam 708 is used for laser beam machining and the like.
FIG. 8 shows a general structure of an optical bench portion of the axial-flow type laser oscillator of the prior art. Output mirror 807 is held in position by output side mirror retainer 815a, and final stage mirror 806 is held in position by final-stage side mirror retainer 815b. Mirror retainers 815a and 815b are provided with cooling plates 816a and 816b respectively, and coolant 817 keeps flowing through cooling plates 816a and 816b to remove heat at all the time. Temperature of coolant 817 is approximately 18° C., and it is introduced into the laser oscillator at a flow rate of approx. 100 l/min. from cooling system 818 provided outside of the laser oscillator.
By the way, there are two states of operation of the laser oscillator when differentiated in a general sense. They are a state in which electrical discharge takes place inside discharge space 805 and another state in which no electrical discharge is produced. It is general practice to produce electrical discharge when laser beam needs to be generated, and the electrical discharge is ceased when the laser beam is not needed.
The laser oscillator operates blower unit 813 to run continuously to keep circulation of the laser gas regardless of using or not using the laser beam, and it generates the electrical discharge each time when it produces the laser beam. It operates blower unit 813 to circulate the laser gas at all the time because it requires several tens of seconds to restart again once blower unit 813 is turned off. On the contrary, it requires only about several tens of milliseconds to stop and to restart the electrical discharge, which is an acceptable level for practical use without a problem.
Although most of the laser beam is reflected by or penetrate through final stage mirror 806 and output mirror 807, a small portion changes to heat due to absorption in them. Final stage mirror 806 and output mirror 807 generate heat when electrical discharge takes place, but they do not heat up when there is no electrical discharge because laser oscillation does not occur.
When heat is generated, they need to be cooled with coolant 817. In the actual practice, however, final stage mirror 806 and output mirror 807 are cooled at all the time while the laser oscillator is in operation regardless of generating or not generating the electrical discharge, since the cooling operation itself is not a problem even when there is no heat.
However, a problem arises when the laser oscillator is used under such an environment as high temperature and high humidity that the components being cooled collect dew condensation. While a small amount of dew condensation does not pose a problem for the regular components, it gives a serious problem for final stage mirror 806 and output mirror 807. No dew condensation occurs on final stage mirror 806 and output mirror 807 when they heat up in the presence of electrical discharge. However, they do collect dew condensation when there is no electrical discharge to produce heat in them. The dew condensation, if formed on any of final stage mirror 806 and output mirror 807, increases absorption factor of the laser beam in the condensed area, which can result in damage to the mirror, and reduction in laser output.
FIG. 9 shows a general structure of another example of the axial-flow type gas laser oscillator of the prior art. Discharge tubes 901 made of a dielectric material such as glass, electrodes 902 and 903 provided on the perimetric sides of discharge tubes 901, power supplies 904 connected to electrodes 902 and 903, discharge spaces 905 inside discharge tubes 901 provided between electrodes 902 and 903, final stage mirror 906, output mirror 907, laser gas passage 910, heat exchanger 911, another heat exchanger 912 and blower units 913 correspond respectively to discharge tubes 801 made of a dielectric material such as glass, electrodes 802 and 803 provided on the perimetric sides of discharge tubes 801, power supply 804 connected to electrodes 802 and 803, discharge spaces 805 inside discharge tubes 801 provided between electrodes 802 and 803, final stage mirror 806, output mirror 807, laser gas passage 810, heat exchanger 811, another heat exchanger 812 and blower units 813 shown in FIG. 8. In addition, a direction of laser beam 908 and flow direction 909 of laser gas also correspond to a direction of laser beam 708 and direction 709 of the laser gas in FIG. 7 respectively.
Blower unit 913 produces a gas flow of approximately 100 m/sec in discharge spaces 905. Inverter 913a controls a driving frequency for rotation of a propelling wheel of blower unit 913.
Laser gas deteriorates over time because it is dissociated by the electrical discharge. Therefore, gas discharge mechanism 915 discharges a certain amount of the laser gas at all times from laser gas passage 910, and gas supply mechanism 916 continues to supply fresh laser gas from the outside to replace the amount of discharged gas. A gas pressure inside the laser gas supply passage is monitored at all the time with gas pressure sensor 917. Gas pressure sensor 917, gas discharge mechanism 915 and gas supply mechanism 916 are connected to gas pressure controller 918. Gas pressure controller 918 maintains the gas pressure in the laser gas passage constant at all the time by controlling gas discharge mechanism 915 and gas supply mechanism 916.
However, conventional axial-flow type gas laser oscillator of the kind described above has problems, which will be discussed hereinafter.
FIG. 10A through FIG. 10C show electric current characteristics of an ordinary type motor used in any of blower units 713, 813 and 913.
FIG. 10A shows a relation between temperature of gas suctioned into any of blower units 713, 813 and 913 and electric current to the motor. Abscissa 1001 represents temperature of the gas suctioned into blower units 713, 813 and 913, and ordinate 1002 represents the electric current that flows to the motor. Line 1003 shows the relation between them.
As is obvious from FIG. 10A, the lower the temperature of the gas suctioned into blower units 713, 813 and 913, the larger the current drawn by the motor of blower units 713, 813 and 913. This is because a mass per unit volume of the gas increases with decrease in temperature of the gas, which increases both the mass and flow rate of the gas delivered per each time period from blower units 713, 813 and 913, which hence increases workload of the motor.
FIG. 10B shows a relation between pressure of the gas suctioned in blower units 713, 813 and 913 and electric current to the motor. Abscissa 1011 represents pressure of the gas suctioned into blower units 713, 813 and 913, ordinate 1012 represents the electric current that flows to the motor, and line 1013 represents the relation between them.
As shown in FIG. 10B, the higher the gas pressure to blower units 713, 813 and 913, the larger the electric current drawn by the motor. A reason of this is that a mass per unit volume of the gas increases with increase in gas pressure, which increases both the mass and flow rate of the gas delivered per each time period from blower units 713, 813 and 913, and it hence increases workload of the motor.
FIG. 10C shows a relation between driving frequency and electric current to the motor of blower units 713, 813 and 913. Abscissa 1021 represents the driving frequency of blower units 713, 813 and 913, ordinate 1022 represents the electric current to the motor, and line 1023 represents the relation between them.
As is apparent from FIG. 10C, the higher the driving frequency of blower units 713, 813 and 913, the faster the rotating speed of a propelling wheel in blower units 713, 813 and 913, and thereby the greater the workload to the motor, which also increases the current drawn by the motor.
In general, increase in the motor current of blower units 713, 813 and 913 increases heat generated in the motor, which results in temperature rise of the motor. In light of the long-term reliability, it is desirable to use a blower unit with as low an amount of motor current as practically possible, since high temperature of the motor accelerates partial deterioration of a motor coil and the like if used continuously for a long period of time.
Normally, the gas pressure inside laser gas passages 710, 810 and 910 is regulated to a predetermined pressure (e.g., approx. 20 kPa) within a range, which can provide an optimum mass and flow rate of the gas while restricting an increase in the amount of current that flows to the motor of blower units 713, 813 and 913. In addition, temperature of the gas suctioned into blower units 713, 813 and 913 is controlled to be about 40 to 50° C. under the normal operating condition, in consideration of balancing between temperature of the laser gas heated during compression by blower units 713, 813 and 913 and heating by the electrical discharge, and cooling capacities of heat exchangers 711, 712, 811, 812, 911 and 912.
Problems are not anticipated so long as blower units 713, 813 and 913 are operated under the above condition at all the time, since the amount of current to the motor is restricted to a certain limit or below, approx. 36 amperes or less for instance. However, another problem comes up in a situation where temperature around the gas laser oscillator decreases in winter or for other reasons. In most cases, the gas laser oscillator is operated only in the daytime, while it is kept not operational during the night hours. The ambient temperature goes down to 5 to 10° C., for instance, when the laser oscillator is not operating during the nighttime in winter. Therefore, temperature of the laser gas inside the gas laser oscillator also goes down to as low a temperature as about 5 to 10° C. by the time the gas laser oscillator is started in the morning. When blower units 713, 813 and 913 are driven under this condition, an amount of current to the motor goes up temporarily to approx. 40 A as compared to the regular level of about 36 A, because temperature of the gas being suctioned in blower units 713, 813 and 913 is low.
In reviewing further detail pertaining to temperature control of the gas suctioned in blower units 713, 813 and 913, it is a general practice that the gas temperature is controlled for cooling only, simply with heat exchangers 711, 712, 811, 812, 911 and 912. Any of heat exchangers 711, 712, 811, 812, 911 and 912 exchanges heat between the gas and cooling water brought in from the outside. Since temperature of the cooling water introduced from the outside is generally in the neighborhood of 15 to 20° C., it can cool the gas having temperature above 15 to 20° C. However, it cannot heat the gas if the temperature is about 5 to 10° C. In the normal operating condition, the gas temperature eventually settles to an expected level of approx. 40 to 50° C. within 10 to 20 minutes even if the gas laser oscillator is started in the low temperature condition with its gas temperature at around 5 to 10° C., because the gas is heated by the heat generated by electrical discharge and compression of the gas by blower units 713, 813 and 913, and the temperature of the motor of blower units 713, 813 and 913 decreases into a normal state without problem. However, the blower unit is operated with the motor consuming a larger current than the anticipated level for a period of about 10 to 20 minutes immediately after the start-up. When the gas laser oscillator is operated everyday in this manner, partial deterioration of motor coils and the like advances in blower units 713, 813 and 913, which consequently leads to a loss of reliability in the long-term use.